A “measuring device” in the sense of this invention is not to be understood as a single device, but rather as a combination of a higher-level unit, such as a transmitter, and a consumer, such as a sensor. The sensor is connected by a cable to the transmitter and supplied with energy by the cable. Moreover, data are exchanged bi-directionally between the sensor and transmitter. Some of the tasks of the transmitter can be transferred to the sensor, i.e., one or more electronic switches such as a microcontroller are located within the sensor. Sensor-specific data such as a name, serial number, production date, device data, calibration data, firmware version, manufacturer information, device driver information, sensor data, history data, and process data can be saved in the microcontroller or in a memory of the sensor. The evaluation of the measured data of the sensor can also be divided between the transmitter and sensor. Furthermore, the addressed tasks can also be taken over by the cable, i.e., “intelligence” can also be located in the cable, such as in the form of a microcontroller that, for example, is located within a cable sheath of the cable. A measuring device supplies a measured value that, for example, corresponds to a distance.
Radar measuring devices that work with microwaves are used in numerous branches of industry, such as the chemical industry, the food industry, or the oil and gas industry. Typically, the fill level in a container needs to be measured. These containers normally have an opening in which a connecting piece or flange is provided for attaching radar measuring devices.
Radar measuring devices are designed to determine the distance of an object, such as the fill level in a container, by transmitting a transmission signal consisting of microwaves by means of a transmission unit to the surface of the object, and receiving as a reception signal the echo waves that are reflected from the surface. An echo function representing the echo amplitudes as a function of the distance is formed, from which a probable useful echo and its runtime are determined. The distance between the object's surface and the transmission unit is determined from the runtime.
With radar measuring devices, the measuring precision can be increased by taking into account an actual phase position at the echo peak of a useful echo function. An abrupt change of the measured value supplied from the processing of the measurement signal is calculated as the phase jump. In the context of the invention, the term “abrupt” is to be understood as a rate of change of a measured value that cannot correspond to a rate change of a spatial position of the surface of an object to be measured relative to the general framework provided by reality. A phase jump is accordingly an error in the evaluation of a measurement signal in which the measured value changes very quickly over time. Phase jumps can occur while processing phase information that is contained in the measurement signal.
Taking into account the actual phase position involves adding up the number of phase periods or wavelengths between the radar measuring device and the surface of the object, as well as the portion of the last phase period or wavelength determined by the actual phase position, in order to precisely calculate the overall measuring distance. A rough measuring distance is accordingly captured from runtime information contained in the intermediate frequency signal, in order to ascertain the number of phase periods between the radar measuring device and the last phase period. Then, a correction of the roughly captured measuring distance is carried out using the phase information contained in the intermediate frequency signal.
In radar measurements, interference signals or briefly occurring measuring conditions can arise that impair the precision in determining the measured distances from the runtime information. Furthermore, it can occur that a radar measuring device has a systematic error in processing the measurement signal. Such disturbances and/or systematic errors can cause the last phase period to be misidentified.
For example, a radar measuring device that serves to measure the level of a medium in a container is installed in a surge pipe. Dispersive effects arise, is in particular, in radar measuring devices that use an FMCW radar-measuring method. When measuring the fill level in a pipe, a frequency-modulated radar signal is coupled into the pipe in accordance with the principles of the FMCW radar. Since the frequency of the coupled-in FMCW radar signal varies, the propagation speed of the radar signal also varies. The different propagation speeds cause the reflected radar signal to blur and dissipate, which must be evaluated by the radar measuring device. This dependence of the propagation speed upon frequency is termed dispersion. The effects caused by the frequency-dependent propagation speed—in particular, the blurring and dissipation of the reflected signal—are termed dispersive effects.
The radar signal propagation speed also varies, depending upon the surge pipe geometry. If the surge pipe diameter is known, the dispersive effects can be taken into account when processing the measurement signal. Consequently, the surge pipe diameter is indicated when the radar measuring device is installed in a surge pipe. It can, however, happen that the indicated surge pipe diameter does not correspond to the actual surge pipe diameter. For example, a surge pipe can have a manufacturing defect such that the actual diameter deviates from the indicated diameter. Such defects can also occur at points in the surge pipe, so that the surge pipe diameter is not constant.
A radar measuring device that is calibrated for a first—in particular, specified—surge pipe diameter and is used in a surge pipe that has a second surge pipe diameter that, in particular, deviates from the first surge pipe diameter, and/or has a varying surge pipe diameter, assigns a systematic error in the processing of the measurement signal. Such errors cause the phase progression of the measurement signal that is ascertained or calculated beforehand to be incorrectly calculated. The phase progression calculated beforehand is termed a target phase progression, wherein a target phase position can be derived for each given distance or measuring distance from the target phase progression. In the event that the target phase progression is ascertained incorrectly, excessive measuring precision in determining the measured distance with runtime information cannot help prevent phase jumps.
If the target phase progression is ascertained incorrectly, or the measuring precision in determining the rough measured distance is insufficient, it can occur that the last phase period is incorrectly identified. In this case, correcting the determination of the measuring distance with phase information of the intermediate frequency signal can cause a jump in the ascertained measured distance of the corresponding measured value. This jump generally contributes about one-half a wavelength of the microwave signal. In the event that a phase period is incorrectly identified as the last phase period and deviates by more than one phase period from the actual last phase period, the measured values corresponding to an ascertained measured distance can also jump to an integral multiple of one-half a wavelength of the microwave signal. Stated more precisely, the jump in the measured value contributes precisely one integral multiple of one-half a wavelength, plus or minus a phase correction factor that is ascertained from the difference between the target phase position and actual phase position.
Conventionally, these jumps have been suppressed through referencing additional information. This additional information is, for example, an index correction table that is conceived for measuring the fill level in a container. An index correction table can be compiled while performing a complete filling and draining process. The jumps that arise in this controlled process can be taken into account or eliminated in measuring mode. A method of this type is described in the document, “FMCW Radar System with Additional Phase Evaluation for High Accuracy Range Detection” by Serdal et al. In this method, changes in a measured value with reference to the preceding measured values ascertained by signal evaluation are considered when evaluating the measurement signal. Abrupt changes in the measured values are then permitted or rejected. Changes arising from phase jumps can thereby be avoided.
Such solutions generally require not only a large memory capacity, but also only become effective after time-consuming procedures have been carried out, such as filling and draining procedures. In addition, the effort must be repeated, if the power fails or the radar measuring device is restarted.